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Magnonics is a research field complementary to spintronics, in which the quanta of spin waves (magnons) replace electrons as information carriers, promising less energy dissipation. The development of ultrafast nanoscale magnonic logic circuits calls for new tools and materials to generate coherent spin waves with frequencies as high, and wavelengths as short, as possible. Antiferromagnets can host spin waves at THz frequencies and are therefore seen as a future platform for the fastest and the least dissipative transfer of information. However, the generation of short-wavelength coherent propagating magnons in antiferromagnets has so far remained elusive. Here we report the efficient emission and detection of a nanometer-scale wavepacket of coherent propagating magnons in antiferromagnetic DyFeO3 using ultrashort pulses of light. The subwavelength nanoscale confinement of the laser field due to large absorption creates a strongly non-uniform spin excitation profile, thereby enabling the propagation of a broadband continuum of coherent THz spin waves. The wavepacket features magnons with detected wavelengths down to 125 nm and supersonic velocities up to 13 km/s that propagate over macroscopic distances. The long-sought source of coherent short-wavelength spin carriers demonstrated here opens up new prospects for THz antiferromagnetic magnonics and coherence mediated logic devices at THz frequencies., Comment: 33 pages, 13 figures

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We propose here a high-frequency spin-Hall nano-oscillator based on a simple magnetic texture, such as a domain wall, located in an antiferromagnet with easy-axis anisotropy type. We show that the spin current, polarized along the anisotropy axis, excites a conical precession of the N\'eel vector in such a domain wall, which allows obtaining a robust ac output signal, -- contrary to the planar precession in an uniform uniaxial antiferromagnet, where ac output is hard to achieve. The frequency of the auto-oscillations is easily tunable by the applied current up to the THz range, and the threshold current vanishes for pure uniaxial antiferromagnet. By micro-magnetic simulations, we demonstrate that the pinning of the domain wall is crucial for the oscillator design, which can be achieved in nano-constriction layout of the free layer., Comment: 7 pages, 5 figures. Brief explanation of the dynamics near the threshold is added at P.3

Ultrafast optical control of magnetic anisotropy in a van der Waals antiferromagnet activates a sub-THz two-dimensional magnon., Van der Waals magnets provide an ideal playground to explore the fundamentals of low-dimensional magnetism and open opportunities for ultrathin spin-processing devices. The Mermin-Wagner theorem dictates that as in reduced dimensions isotropic spin interactions cannot retain long-range correlations, the long-range spin order is stabilized by magnetic anisotropy. Here, using ultrashort pulses of light, we control magnetic anisotropy in the two-dimensional van der Waals antiferromagnet NiPS3. Tuning the photon energy in resonance with an orbital transition between crystal field split levels of the nickel ions, we demonstrate the selective activation of a subterahertz magnon mode with markedly two-dimensional behavior. The pump polarization control of the magnon amplitude confirms that the activation is governed by the photoinduced magnetic anisotropy axis emerging in response to photoexcitation of ground state electrons to states with a lower orbital symmetry. Our results establish pumping of orbital resonances as a promising route for manipulating magnetic order in low-dimensional (anti)ferromagnets.

Excitation of antiferromagnetic HoFeO3 with a single 80 fs laser pulse triggers a first-order spin-reorientation phase transition. In the ultrafast kinetics of the transition one can distinguish the processes of impulsive excitation of spin precession, nucleation of the new domain and growth of the nuclei. The orientation of the spins in the nuclei is defined by the phase of the laser-induced coherent spin precession. The growth of the nuclei is further promoted by heating induced by the laser excitation. Hereby we demonstrate that in HoFeO3 coherent control of the spin precession allows an effective control of the route of the heat-induced first-order magnetic phase transition. The theoretical description of the excitation of the spin precession by linearly-polarized ultrashort laser pulses is developed with the sigma model. The analysis showed high sensitivity of the excited dynamics to the initial spin orientations with respect to the crystallographic axes of the material.

Excitation of the collinear compensated antiferromagnet DyFeO_{3} with a single 60 fs laser pulse triggers a phase transition across the Morin point into a noncollinear spin state with a net magnetization. Time-resolved imaging of the magnetization dynamics of this process reveals that the pulse first excites the spin oscillations upon damping of which the noncollinear spin state emerges. The sign of the photoinduced magnetization is defined by the relative orientation of the pump polarization and the direction of the antiferromagnetic vector in the initial collinear spin state.

Here we study the instability of the vortex motion caused by non-monotonous dependence of the friction force on the vortex velocity in the presence of a gyroforce. The analysis is done within the frameworks of “minimal” model for the elastic string subject to friction force and gyroforce. We demonstrate that even a weak gyroforce renders the condition for onset of instability more strict than for a vortex not subject to gyroforce: either the friction force must exceed a threshold, or its non-monotonic velocity dependence must be a rather steep function.

Submicron permalloy rings can have a vortex ground state as a result of the competing effects arising from the exchange and magnetostatic interactions. The dynamic properties of this state are investigated by analytical calculations of the structure and frequencies of the magnon normal modes in the presence of the vortex–magnon interaction. For thin rings of thickness, L = 60 nm and inner and outer radii, Ri and Ro=200 nm, respectively it is shown that the magnon node frequencies in the 10 GHz range for rings of radii of a few hundred nanometers. Moreover, the frequencies are slowly increasing functions of Ro for fixed Ro.

Both static and dynamic properties of the magnetic dot array with the uniform in-plane magnetization with the most symmetric configuration (round dots placed in a square lattice) have been investigated. Due to the long distance dipole coupling physical properties, such system has a strong four-fold anisotropy with respect both to orientation of the total array magnetization regarding the lattice symmetry axis and to the wave vector direction of a collective oscillations wave. The ferromagnetic state is stable only in the presence of the magnetic field; however, it can also be stable against small perturbations for some orientations of the total magnetization.

The effect of a strong magnetic field H, perpendicular to the easy axis of a ferromagnetic, on the motion of a domain wall is investigated. It is shown that the peak velocity of motion vc is defined by various scenarios at weak (H≪Ha) and strong (Ha - H ≪ Ha) magnetic fields, where Ha is the anisotropy field. Therefore, the peak velocity Vc is a non- monotonous function of the magnetic field, when H ranges from 0 to Ha. The character of the DW velocity dependence on the driving field Hz was studied.

The peculiar dynamical drag force F/sub c/ can arise at the domain wall motion in a magnet with microscopic defects, such that it cannot pin the domain wall in the usual way. The value of F/sub c/ is finite as the domain wall velocity v goes to zero. The reason for such behavior is the emission of bending waves of the domain wall. The calculations for different types of impurities as well as for both bulk ferromagnet and ultrathin magnetic layers are presented. In 3D ferromagnet the F/sub c/ value is finite at v/spl rarr/0 and this is interpreted as the dynamical coercive force. In the 2D case (e.g. in monolayer films) the drag force diverges at small velocities as F/sub c//spl prop/1/v/sup 1/2/. In the film with a finite thickness and very small v the force has a viscous character: F/sub c//spl prop/v. >

In the framework of a generalized magnetic relaxation phenomenological theory, which includes the exchange relaxational term, the Bloch point (BP) dissipative dynamics was investigated. It was shown, that consideration of exchange relaxation can explain the extremely small BP mobility value, observed in the experiment, contrary to conventional relaxation theory, based on relativistic terms.

Relaxation of spin waves and domain walls in antiferromagnets has been analyzed within the framework of microscopic and phenomenological approaches. Decrement of the magnon damping and the coefficient of the domain wall viscosity are calculated. The relaxation constants of relativistic and their temperature dependencies are calculated based on comparison of the two approaches. Limitations of the phenomenological approach usability are discussed.

The question of kink-type (domain-wall-type) topological soliton motion with a kink interaction with the elastic deformation field was considered. The viscosity of the deformation field was taken into account. It is shown that kink motion with a velocity in some interval around the sound velocity is impossible for some relations between the system parameters (elastic gap effect). For small coupling constants or a large viscosity the elastic gap does not exist. The results can describe the domain wall dynamics in magnetics or the proton kink dynamics in systems with hydrogen bounds.

The kink (domain wall) structure in the ferromagnet with the spin S = 1 and extremely high anisotropy (when the spin quantum reduction is not small) is investigated. The consideration is carried out at low temperatures and near the quantum phase transition from the ferromagnetic phase to the quadruple one where the magnetization is zero. Two kink types can exist in this region: a usual kink with the magnetization turn and the kink where only the value of the magnetization vector changes. The spin wave spectrum on the background of the kink is found using the Hubbard operator technique. Phenomenological equations describing the magnetization dynamics in the ferromagnet of this kind are proposed on the basis of the modification of static equations for a mean spin; magnons spectra are calculated. The dynamic properties of the kinks of both types are investigated on their basis.

The interaction between a ferromagnetic monolayer and nonmagnetic substrate is investigated. The characteristic dimension of superstructure arising in a film is estimated. New types of surface magnetoelastic waves are obtained near the 2D phase transition of a homogeneous magnetic state to the state with surface superstructure.

In the magnet models, which provide the small-radius static topological solitons, the existence of magnetic vortices is considered. The latter vortices can be represented as microscopical magnetic bubbles with radius less than the Bloch wall thickness in the magnet under consideration. The vortices dynamics is investigated and proof of their stability is given.

A microscopic theory of domain wall mobility in the material of iron-garnet type with rare-earth ions has been formulated. The domain wall mobility appeared to be due to two mechanisms (so-called transverse and longitudinal relaxation) known from the theory of ferromagnetic resonance line width. The theory allows the domain wall mobility to be expressed through normal parameters of the material (anisotropy and exchange energy), those of the rare-earth ion, ion concentration and temperature. Microscopic estimation easily accounts for the difference between the relaxation constants measured in the experiments on magnetic resonance and from the mobility of domain walls.

The dispersion relations for collective magnon modes for square-planar arrays of vortex-state magnetic dots, having closure magnetic flux, are calculated. The array dots have no direct contact between each other, and the sole source of their interaction is the magnetic dipolar interaction. The magnon formalism using Bose operators along with translational symmetry of the lattice, with the knowledge of mode structure for the isolated dot, allows the diagonalization of the system Hamiltonian giving the dispersion relation. Arrays of vortex-state dots show a large variety of collective mode properties, such as positive or negative dispersion for different modes. For their description, not only dipolar interaction of effective magnetic dipoles, but nondipolar terms common to higher multipole interaction in classical electrodynamics can be important. The dispersion relation is shown to be nonanalytic as the value of the wave vector approaches zero for all dipolar active modes of the single dot. For vortex-state dots, the interdot interaction is not weak, because the dynamical part (in contrast to the static magnetization of the vortex state) dot does not contain the small parameter, the ratio of vortex core size to the dot radius. This interaction can lead to qualitative effects like the formation of modes of angular standing waves instead of modes with definite azimuthal number known for the insolated vortex state dot.

A new mechanism of coercivity is valid when the characterizing scale of space inhomogeneities with a size much less than the domain wall (DW) width and when the traditional mechanism is not valid. It is connected with the excitements of the DW vibrations because of interaction with inhomogeneities. The coercive fields in amorphous and nanocrystalline soft materials are calculated.

The mechanisms of the domain wall relaxation in the ferromagnets have been analyzed in the frameworks of the phenomenological approach. It is shown that the mechanisms considered previously can not explain the experimental data entirely. The new diffusion mechanisms of the domain wall damping is discussed.

The dynamical properties of topological precessional solitons in two-dimensional ferromagnets with uniaxial anisotropy are considered. It turns out that free (without external forces acting) soliton motion corresponds to some rotary motion (analogous to Larmor precession of a charged particle in an external magnetic field). The effective mass of the soliton is found. It is shown that the denominator of the mass value contains the anisotropy constant, so in the isotropic ferromagnet the topological soliton motion is impossible.

The study is made of soliton solution of nonlinear equations describing dynamics of some magnets as well as the field theory models. Three-dimensional solitons are analysed to which non-uniform magnetization precession corresponds. Necessary and sufficient conditions for soliton stability according to Lyapunov are obtained. The soliton without nodes is proved to be stable over a wide range of the precission frequencies and unstable beyond this range. Solitons with nodes are unstable at all frequencies.

A new scheme of structural-facial and tectonical subdivision of South Sikhote-Alin is proposed. The whole of the territory is subdivided into three structural-facial zones: Grodekovo, Khankay and Sikhote-Alin. The Grodekovo zone occupies the western part of the Primorye region, shows a meridional strike and is distinguished from other zones by a wide distribution of thick volcanic and volcanic-detrital Permian deposits of the geosynclinal type, an abundance of great upper Paleozoic granitoid intrusions and continental facies of Mesozoic and Cenozoic sediments. The Sikhote-Alin zone of the Sikhote-Alin geosyncline proper represents a complicated structure in consequence of repeated, sometimes intensive tectonic movements, resulting in an unequal deformation of different parts of the geosyncline; the downwarping of the latter was widespread and continuous at one time and differentiated and quickly replaced by uplifts at another. The Sikhote-Alin geosyncline deposits are grouped as follows: schistose (Devoni...

A classification of topological solitons in spin glasses is made within the macroscopic description. It is shown that the stable dynamic solitons of non-topological character can exist in the spin glass.

Magnetic solitons in a one-dimensional ferromagnet with easy-plane anisotropy are considered. Algebraic solitons are discussed and the conditions of existence of such solitons are investigated.